The vapor pressure graph is a fundamental scientific tool that visually links a substance’s temperature and the pressure exerted by its gaseous phase. It allows scientists to predict when a liquid will transition into a gas under various conditions. Understanding how to read this curve is a direct method for analyzing a substance’s physical properties and determining its boiling temperature without conducting a physical experiment.
The Relationship Between Vapor Pressure and Boiling
Vapor pressure is the force exerted by vapor molecules above a liquid, representing the tendency of a substance to become a gas. This pressure increases as the temperature rises because the molecules gain energy and escape into the gaseous phase more readily. The boiling point is the specific temperature at which a liquid turns into a gas throughout its bulk, forming bubbles.
Boiling occurs when the liquid’s vapor pressure becomes equal to the external pressure pushing down on the liquid’s surface. Once the internal vapor pressure matches the external pressure, the liquid can boil freely. The normal boiling point is a standardized value defined as the temperature at which the vapor pressure equals one standard atmosphere (atm), or 760 millimeters of mercury (mmHg). This standard provides a consistent reference point for comparing different substances.
Reading the Vapor Pressure Curve
A vapor pressure curve is a graph plotting the vapor pressure of a substance against its temperature. The horizontal X-axis represents the temperature, typically measured in degrees Celsius, while the vertical Y-axis represents the vapor pressure, often in units like atm or mmHg. The plot for any given substance is not a straight line but an upward-curving, exponential shape.
This curved shape shows that vapor pressure does not increase linearly with temperature but accelerates as the substance gets hotter. A single graph often contains multiple curves, with each line representing a different liquid. More volatile substances, which evaporate easily, show a steeper curve and sit higher on the graph. Substances with stronger molecular attractions require higher temperatures to achieve the same vapor pressure, resulting in a curve that is lower and shifted toward the right.
Determining Boiling Point Step-by-Step
Finding the normal boiling point of a substance involves a four-step process using the vapor pressure curve.
- Locate the standard atmospheric pressure (1 atm or 760 mmHg) on the vertical Y-axis of the graph. This pressure value is the universal reference point for defining the normal boiling temperature.
- Draw a straight horizontal line across the graph from the 1 atm mark. This line represents the condition where the vapor pressure equals the standard external pressure.
- Identify the exact point where this horizontal pressure line intersects the specific curve for the liquid being analyzed. This intersection point represents the substance being at 1 atm pressure and its normal boiling temperature.
- Drop a vertical line straight down from this intersection point to the horizontal X-axis. The temperature value read on the X-axis at this point is the substance’s normal boiling point.
How Changing External Pressure Affects the Result
The utility of the vapor pressure curve extends beyond finding the normal boiling point, as it can predict the boiling point at any external pressure. Since boiling occurs when vapor pressure equals the surrounding pressure, a change in the external environment directly alters the boiling temperature.
For instance, at high altitudes, the atmospheric pressure is lower than 1 atm, meaning the liquid needs less internal pressure to boil. To find this boiling point, the horizontal line is drawn at the lower atmospheric pressure reading specific to that altitude on the Y-axis. This lower pressure line intersects the substance’s curve at a lower temperature, confirming that the boiling point decreases with altitude. Conversely, in a sealed container like a pressure cooker, the external pressure is intentionally increased, requiring the liquid to reach a higher temperature before its vapor pressure can match the surrounding pressure.